The conditions giving rise to the problems solved by this invention are commonly found in industries utilizing externally driven valves. In particular within the power industry, valves are operated remotely from open, closed and intermediate positions to improve or maintain utility power plant output, or in many cases provide for the protection of the general public from release of radioactive materials either directly or indirectly. Continual proper operation of these valves is essential to industries and the general public.
Typically these valves are required to operate under differing operation conditions of temperature, pressure and flow within the common requirement for consistent operation. Further, the inherent operating characteristics of the valve and operator are constantly undergoing mechanical or electrical changes from maintenance, repair, adjustments, calibration and wear.
In the earlier state-of-the-art, remote and local externally operated valves have been tested and calibrated to demonstrate that the operator will deliver the minimum or maximum thrust loads to the valve stem under static conditions. The prior state-of-the-art did not provide verification that the static load delivered by the operator was acceptable after field assembly or maintenance, nor did the prior art provide any verification of the imposed valve load under dynamic conditions.
Historically, the thrust required to open or shut and subsequently to seat a valve was determined analytically by considering such factors as temperature, pressure, pressure drop, flow, liquid, valve type, packing load, motor voltage and valve mechanical characteristics. Once the minimum and maximum valve thrust requirements were determined analytically, the valve operator size could then be selected. Normally, motor operated valves in nuclear power plants, which perform a safety function, are required to operate between seventy-five and one hundred-ten percent (75-110%) of nominal line voltage applied to the operator. This requirement could lead to sizing of operator motors which can deliver from 1.0 to 2.5 times the required valve stem thrust, depending upon the actual line voltage. With oversized motors and operators, the load imposed on the valve is typically much larger than anticipated or estimated by static means, due to the inherent motor/operator inertia effects. Prior state-of-the-art methods are minimizing the effects of dynamic or inertia forces involved the use of torque switches, motor brakes or compensating springs. Although these devices provided some relief, they do not preclude excessive or inadequate thrust loads being delivered to the valve stem, seat and body. Complicating this situation is the fact than when a valve leaks, common practice has been to increase the force delivered to the valve stem through increased torque switch adjustments. Studies have shown that this approach subsequently leads in many cases to irrevocable damage to the valve or inoperativeness and more importantly degradation of system reliability as a whole.
The basic shortcoming of the prior art-load limiting devices is that they are not diagnostic on nature and, as in the case of the torque switch, provide an element of protection which does not take into account the dynamic considerations of the valve and operator during actual operation. Changing effects on valve load under dynamic conditions such as line voltage, packing tightness, gear train wear, lubrication degradation, calibration, and adjustment errors cannot be identified with the earlier state of the art devices. Further, in most cases, prior state of the art post-maintenance valve and operator actuation testing will not identify progressive degradation of valve performance.